According to a known rotor yoke of an inner rotor motor, a rotor is configured by mounting a rotation shaft and a field pole magnet (hereinafter referred to as a magnet) to the rotor yoke. Because the inner rotor motor has a structure that the rotor is arranged at an inner side of the motor and a stator wound with a coil is arranged at an outer side of the rotor, the inner rotor motor has characteristics that, for example, a moment of inertia of the rotor is small and heat dissipation of the coil is preferable. Normally, the magnet is not magnetized before being mounted to the rotor yoke, and is magnetized by a magnetizing yoke after the magnet is mounted on the rotor yoke. When magnetizing the magnet, in order to apply desired magnetic properties to the magnet, a positioning of the rotor yoke with the magnet mounted against the magnetizing yoke needs to be precise.
A known electric motor rotor is disclosed in JP2010-236453A (hereinafter referred to as Patent reference 1). According to the Patent reference 1, an electric motor rotor including a substantially columnar rotor laminated steel plate internally having a permanent magnet is disclosed. The rotor laminated steel plate of the electric motor rotor is formed with plural refrigerant gas holes axially penetrating, and plural rivet holes circumferentially arranged and axially penetrating. At least one of the plural rivet holes is used as a positioning hole for inserting a positioning pin to position a rotor in a rotational direction when the rotor is magnetized.
According to the rotor laminated steel plate of the Patent reference 1, plural magnets are arranged circumferentially. Each magnet extends in an axial direction over an entire length thereof. The rivet holes are arranged between the magnets which are adjacent to each other and slightly radially inner of the magnets. However, the rivet holes are arranged at a portion of a magnetic circuit (hereinafter also referred to as a magnetic path) generated when the motor is operated as the electric motor. This arrangement decreases magnetic efficiency of the motor because the rivet holes act as magnetic resistance.
Because the rotor laminated steel plate exists between the magnets adjacent to one another in a circumferential direction, magnetic field lines are leaked between the magnets adjacent to one another in the circumferential direction from a magnetic path formed between the rotor laminated steel plate and the stator. Thus, magnetic efficiency of the motor is decreased.
A need thus exists for a rotor yoke and a motor applying the rotor yoke which is not susceptible to the drawback mentioned.